Origin of the earth

A global outside is covered up by several plates, and is a huge jigsaw puzzle.
Present Japan is in the location where 4 of oceanic plate (Taiheiyoupureto and the Philippine Sea plate) touched continental plate (Eurasian plate and North American plate).

The time of STEP1
The era of a deep sea 20-70 million years ago

As oceanic plate was depressed, oceanic plate and the one accumulated in a boundary of continental plate (trench sediments) were forced on land one after another, and a foundation in Nanki Kumano (accretionary prism) was made.

The time of STEP2
The era of a shallow sea 15-18 million years ago

A fold was forming on the accretionary prism which was forced on land and became shallow one after another, and the sand and the mud which have flowed from the land were accumulated (forearc basin sediments).

The time of STEP3
The era of a intense volcanic activity 14-15 million years ago

Magma activity became active, and large-scale volcano (igneous rocks) could penetrate accretionary prism and a forearc basin sediments was made.

The time of STEP4 Three geological features form the present platform

In Nanki  Kumano region, there are 3 distributed geological parts, an accretionary prism made of accumulated stratum in ocean trench, forearc basin sedimens on the accretionary prism, igneous rocks composed of magma. Current terrain resulted from diastrophism and weathering erosion in a newer era.




In purpose of the person who would like to know more conversantly.

The earth in Nanki Kumano is made of 3 geological bodies made with depression of a plate (accretionary prism, forearc basin sediments and igneous rocks).

The time of STEP1 The era of a deep sea 20-70 million years ago

The stratum accumulated in the ocean area and the earth and sand carried by erosion weathered in land to a sea are quite deformed by a shake of a big earthquake, a flood and violent wave, and the trench sediments of which accretionary prism is composed is submarine mudslide, is turbidity current and flows through the bottom of the sea, and is the one which got off, formed submarine fan in a trench and accumulated (60-70 million years ago Hidakagawa Group Ryujin accretionary prism, 50-60 million years ago Otonashigawa Group and 20-50 million years ago Muro Group). It was forced on landward slope of trench underground hard and was accretionary prism with depression of a plate just after the accumulation, and lifted and returned to land again, and it became the earth.

Phoenix Fold(Susami-cho)

Phoenix Fold(Susami Town)


The time of STEP2  The era of a shallow sea 15-18 million years ago

The earth and sand where a forearc basin sediments was also carried by erosion weathered to a mouth of a river likewise were carried to a fold in the shallow sea which has formed on the accretionary prism, and an accumulated stratum (15-18 million years ago) was pressed up with protuberance of accretionary prism and became the earth.

Senjojiki RockPlateau(Shirahama-cho)

Senjojiki RockPlateau(Shirahama Town)


The time of STEP3 The era of a intense volcanic activity 14-15 million years ago

step3 Igneous rocks could dissolve the part of the mantle and the accretionary prism depth in depressed movement of a plate, and became a magma and rose and solidified cooling and became the earth (14-15 million years ago).

Ichimai-iwa Monolith(Kozagawa-cho)

Ichimai-iwa Monolith(Kozagawa Town)

Gotobiki-iwa Rock(Shingu-shi)

Gotobiki-iwa Rock(Shingu City)

The tame of STEP4 Three geological features form the present platform

These geological bodies show its form in the surface of the earth at present by protuberance in Kii Peninsula and influence of erosion.


Kii Peninsula is the area rose by the shape that the earth which consists of 3 geological bodies undergoes depressed influence of a plate and sticks out in a Kuroshio Current. Therefore steep mountain and vast sea are next to each other. For water vapor from the Pacific Ocean to be crashed against a steep mountain, precipitation is the damp climate much mildly. It depends every the remarkable bottom of many rainfall and branch, and a deep valley is formed and it’s difficult for plains to develop, since dropping in, there is also a place where a river terrace, the ring way hilly area and a small-scale alluvial plain exist around the river.

Kii Peninsula rises as the whole, but it’s possible to see the submergence topography in a part. The coastal terrace made with protuberance in the earth and sea level change in a glacier with a disastrous earthquake can remain as the topography as “Hirami” as well as see wide wave-cut bench around the coast in the area. The submergence topography of the submerged valley and Aegean Sea exists, and it’s possible to judge various seashore cityscape by complication.

Investigated by IODP (integration international deep-sea drilling plan) using a deep earth research vessel “the earth” at Kumano-nada in the Kii Peninsula offing at present, and the structure of the accretionary prism which is being formed at present and a movement of a plate at a subduction zone are being elucidated. This study is to advance it as a study and unity of Shimanto Accretionary Prism in Kii Peninsula, and you expect that elucidating a mechanism of accretionary prism formation in detail and also contribute to explication of a disastrous earthquake by plate subduction zone. It’s possible to observe the structure of the mud diapiric fold paid much attention by a methane hydrate investigation in the land in the huge forearc basin deposits of Nanki Kumano, and it’s expected as a valuable source of resource-searching.